U.S. patent number 4,574,263 [Application Number 06/742,365] was granted by the patent office on 1986-03-04 for infrared radiation detector.
This patent grant is currently assigned to The Commonwealth of Australia. Invention is credited to Kevin C. Liddiard.
United States Patent |
4,574,263 |
Liddiard |
March 4, 1986 |
Infrared radiation detector
Abstract
An infrared radiation detector in which a stable substrate (5)
has a hole (10) through it and a pair of bonding pads (9) on two
opposite sides of the hole, the hole being covered by a pellicle
(6) of insulating or semi-conductor material with supports over the
hole, a detector element (7) comprising an ultrathin infrared
absorbing film of nickel, palladium, platinum or iridium less than
10 nm thick or a gold film less than 20 nm thick, two sides of the
detector element connecting to the bonding pads (9) by thin film
contacts (8).
Inventors: |
Liddiard; Kevin C. (Fairview
Park, AU) |
Assignee: |
The Commonwealth of Australia
(Canberra, AU)
|
Family
ID: |
3768722 |
Appl.
No.: |
06/742,365 |
Filed: |
June 7, 1985 |
PCT
Filed: |
September 17, 1981 |
PCT No.: |
PCT/AU81/00130 |
371
Date: |
May 21, 1982 |
102(e)
Date: |
May 21, 1982 |
PCT
Pub. No.: |
WO82/01066 |
PCT
Pub. Date: |
April 01, 1982 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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387852 |
May 21, 1982 |
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Foreign Application Priority Data
Current U.S.
Class: |
338/18;
250/338.1; 250/338.4; 338/25 |
Current CPC
Class: |
H01L
37/02 (20130101); G01J 5/20 (20130101) |
Current International
Class: |
G01J
5/20 (20060101); H01L 37/00 (20060101); H01L
37/02 (20060101); H01L 031/08 () |
Field of
Search: |
;338/18,25 ;250/338 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Gosch, J. "Thin-Film Enhances Balometer's Sensitivity", Electronics
Feb. 28, 1980 pp. 74-76. .
Elliot, D. J., Integrated Circuit Fabrication Technology, McGraw
Book Co. .COPYRGT.1982 pp. 18-21, 24-25, 32-33, and 259. .
Coombe, R. A., The Electrical Properties and Applications of Thin
Films, Sir Issac Pitman & Sons Ltd. .COPYRGT.1967 pp. 88, 133.
.
Bruck et al. in Electronics, vol. 51, No. 16, pp. 99 to 104,
1978..
|
Primary Examiner: Albritton; C. L.
Assistant Examiner: Lateef; M. M.
Attorney, Agent or Firm: Cushman, Darby & Cushman
Parent Case Text
This is a continuation of application Ser. No. 387,852, filed May
21, 1982, now abandoned.
Claims
The claims defining the invention are claimed as follows:
1. An infrared radiation detector comprising
(a) a substrate formed of a stable material such as an aluminum
oxide ceramic, a glass, a metal having an electrically insulating
surface, or a silicon slice, said substrate having a hole
therethrough;
(b) a pair of bonding pads spaced apart one on each side of the
said hole;
(c) a pellicle having a thickness less than 50 nm to have a low
heat retaining capacity and high thermal resistance formed of an
insulating or semiconductor material selected from the group
consisting of aluminium oxide, silicon dioxide, silicon monoxide,
silicon nitride, cryolite, germanium, and silicon, positioned to
span the said aperture;
(d) a detector comprised of an essentially straight electrical
resistance element fixed on the said pellicle to be positioned over
the said hole, and comprising an ultrathin infrared-absorbing film
less than 10 nm thick selected to form both the absorber of
infrared energy and a resistance element and selected from the
group consisting of nickel, palladium, platinum and iridium such
metal being selected to have a low heat retaining capacity and high
thermal resistance; and
(e) a pair of thin-film conductive contacts connecting opposite
sides of the said detector element to the said bonding pads.
2. An infrared radiation detector according to claim 1 formed as an
array of detector elements spaced apart on an elongated pellicle
mounted on the said substrate to extend across a series of holes,
each detector element being joined to a pair of thin-film
interformed connects by a pair of thin-film contacts, each detector
of the array and the thin-film contacts and interconnects being
electrically isolated from each adjacent detector element.
3. An infrared radiation detector according to claim 1 or 2 wherein
the substrate is a ceramic and the said holes are formed by a high
speed diamond drill.
4. An infrared radiation detector according to claim 1 or 2 wherein
the substrate is glass and the said holes are formed by
photolithography using an etchant.
5. An infrared radiation detector according to claim 1 or 2 wherein
the substrate is polycrystalline or single crystal silicon, and the
said holes are formed by photolithography using an etchant.
6. An infrared radiation detector according to claim 1 or 2 wherein
the substrate is metal having a polished surface coated with an
insulating layer by anodising.
7. An infrared radiation detector according to claim 1 or 2 wherein
the substrate is metal having a polished surface coated with an
insulating layer by chemical deposition.
8. An infrared radiation detector according to claim 1 or 2 wherein
the substrate is metal having a polished surface coated with an
insulating polymer.
9. An infrared radiation detector according to claim 1 or 2 wherein
the substrate has the said holes formed in it by spark erosion.
10. An infrared radiation detector according to claim 1 or 2
wherein the substrate has the said holes formed in it by abrasive
milling.
11. An infrared radiation detector according to claim 1 or 2
wherein the said detector element prepared by deposition through
precision micro-mechanical masks.
12. An infrared radiation detector according to claim 1 or 2
wherein the said detector element prepared by deposition followed
by pattern etching using photolithography.
13. A method of producing an infrared radiation detector, which
detector comprises a substrate formed of a stable material having a
hole therethrough, a pair of bonding pads spaced apart one on each
side of the hole, a pellicle having a thickness less than 50 nm
formed of an insulating or semiconductor material selected from the
group consisting of aluminium oxide, silicon dioxide, silicon
monoxide, silicon nitride, cryolite, germanium, and silicon,
positioned to span the said aperture, a detector comprised of an
electrical resistance element carried on the said pellicle to be
positioned over the said hole, and comprising an ultrathin
infrared-absorbing film selected from the group consisting of
nickel, palladium, platinum, and a pair of thin-film conductive
contacts connecting opposite sides of the said detector element to
the said bonding pads, said method comprising forming said pellicle
by anodising of vacuum deposition on to a backing comprising a
metal film of polymer membrane while generating the required shape
by masking techniques, pressing said pellicle to the said
substrate, and removing said backing.
14. A method of producing an infrared radiation detector, which
detector comprises a substrate formed of a stable material having a
hole therethrough, a pair of bonding pads spaced apart one on each
side of the hole, a pellicle having a thickness less than 50 nm to
have a low heat retaining capacity and high thermal resistance
formed of an insulating or semiconductor material selected from the
group consisting of aluminium oxide, silicon dioxide, silicon
monoxide, silicon nitride, cryolite, germanium, and silicon,
positioned to span the said aperture, a detector comprised of an
electrical resistance element carried on the said pellicle to be
positioned over the said hole, and comprising an essentially
straight ultrathin infrared-absorbing film less 10 nm thick
selected to form both the absorber of infrared energy and a
resistance element and selected from the group consisting of
nickel, palladium, platinum or iridium, and a pair of thin-film
conductive contacts connecting opposite sides of the said detector
element to the said bonding pads, said method comprising forming
said pellicle by depositing the pellicle material by vacuum or
chemical vapour deposition directly onto a silicon substrate and
then etching said hole into the rear surface of the silicon wafer
whereby to render the pellicle freely-supporting.
15. An infrared radiation detector comprising a substrate formed of
a stable material such as an aluminium oxide ceramic, a glass, a
metal having an electrically insulating surface, or a silicon
slice, said substrate having a hole therethrough; a pair of bonding
pads spaced apart one on each side of the said hole; a pellicle
having a thickness less than 50 nm to have a low heat retaining
capacity and high thermal resistance formed of an insulating or
semiconductor material selected from the group consisting of
aluminium oxide, silicon dioxide, silicon monoxide, silicon
nitride, cryolite, germanium, and silicon, positioned to span the
said aperture; a detector comprised of an essentially straight
electrical resistance element fixed on the said pellicle to be
positioned over the said hole, and comprising an ultrathin
infrared-absorbing film selected to form both the absorber of
infrared energy and a resistance element and formed of a gold film
less than 20 nm thick having a low heat retaining capacity and high
thermal resistance; and a pair of thin-film conductive contacts
connecting opposite sides of the said detector element to the said
bonding pads.
16. A method of producing an infrared radiation detector, which
detector comprises a substrate formed of a stable material having a
hole therethrough, a pair of bonding pads spaced apart one on each
side of the hole, a pellicle having a thickness less than 50 nm to
have a low heat retaining capacity and high thermal resistance
formed of an insulating or semiconductor material selected from the
group consisting of aluminium oxide, silicon dioxide, silicon
monoxide, silicon nitride, cryolite, germanium, and silicon,
positioned to span the said aperture, a detector comprised of an
essentially straight electrical resistance element fixed on the
said pellicle to be positioned over the said hole, and comprising
an ultrathin infrared-absorbing film selected to both the absorber
of infrared energy and a resistance element and formed of a gold
film less than 20 nm thick, an a pair of thin-film conductive
contacts connecting opposite sides of the said detector element to
the said bonding pads, said method comprising forming said pellicle
by anodizing or vacuum deposition onto a backing comprising a metal
film or polymer membrane while generating the required shape by
masking techniques, pressing said pellicle to the said substrate,
and removing said backing.
17. A method of producing an infrared radiation detector, which
detector comprises a substrate formed of a stable material having a
hole therethrough, a pair of bonding pads spaced apart one one each
side of the hole, a pellicle having a thickness less than 50 nm to
have a low heat retaining capacity and high thermal resistance
formed of an insulating or semiconductor material selected from the
group consisting of aluminium oxide, silicon dioxide, silicon
monoxide, silicon nitride, cryolite, germanium, and silicon,
positioned to span the said aperture, a detector comprises of an
essentially straight electrical resistance element carried on the
said pellicle to be positioned over the said hole, and comprising
an ultrathin infrared-absorbing film selected to from both the
absorber of infrared energy and a resistance element and formed of
a gold film less than 20 nm thick, and a pair of thin-film
conductive contacts connecting opposite sides of the said detector
element to the said bonding pads, said method comprising forming
said pellicle by depositing the pellicle material by a vacuum or
chemical vapour deposition directly onto a silicon substrate and
then etching said hole into the rear surface of the silicon wafer
whereby to render the pellicle freely-supporting.
Description
This invention relates to a method of preparation of an improved
high performance thermal infrared detector and to the detector
construction itself.
According to the invention a single detector or array of detectors,
used in conjunction with a suitable optical system, detects the
infrared heat radiation emitted from bodies, but will also operate
over a broad wavelength region, from the near to far infrared.
The detector is of the resistance bolometer type, that is absorbed
infrared radiation raises the temperature of the detector, thereby
causing the electrical resistance to change. This change is
observed by virtue of the variation in an electrical bias current
or voltage applied to the detector.
It must be understood that detectors of this type have been
previously described and reference may be had to the following
papers:
(1) A paper by C. B. AIKEN, W. H. CARTER Jr, and F. S. PHILLIPS
entitled "The Production of Film Type Bolometers with Rapid
Response", published in Rev.Sci.Ins., Vol. 17, No. 10, p. 377,
1946.
(2) A paper by B. H. BILLINGS, W. L. HYDE and E. F. BARR entitled
"Construction and Characteristics of Evaporated Nickel Bolometers",
Rev.Sci.Ins., Vol. 18, No. 6, p. 429. June 1947.
(3) A paper by K. YOSIHARA entitled "An Investigation of the
Properties of Bolometers made by Vacuum Evaporation", Science of
Light, Vol. 5, No. 2, p. 29. 1956.
(4) A paper by W. R. BLEVIN and W. J. BROWN entitled "Large-area
Bolometers of Evaporated Gold", J.Sci.Ins., Vol. 42, No. 1, p. 19,
January 1965.
However, the material properties of these detectors are not
optimum, and it is an object of the present invention to improve
the detecting ability of devices of this type. The invention deals
with the method of preparation and the introduction of new
materials technology.
(5) A recent report by GOSCH under the title "Thin Film enhances
Bolometer's Sensitivity", published in Electronics, 28 Feb. 1980,
page. 75, describes a gold film bolometer which is said to have
exceptionally high performance, but the information given is
incomplete.
Patents on bolometers are also well known, for example, the patent
to R. J. HAVENS et al, U.S. Pat. No. 2,516,873; the patent to J.
LEBLANC et al, U.S. Pat. No. 4,116,063; the patent to P. PAUL, U.S.
Pat. No. 3,745,360; the patent to B. NORTON et al U.S. Pat. No.
3,202,820; and the patent to E. H. EBERHARDT AVS No. 220,305.
However these patents do not describe either the method of
preparation or the materials technology which are features of the
present invention.
It is further understood that the general concept of a bolometer
detector comprised of a thin infrared sensitive film deposited onto
a pellicle which is supported over an aperture is described in the
patent by R. J. HAVEN et al and in the papers cited above, and
consequently is not specifically claimed in the present
invention.
The detector according to this invention is comprised essentially
of an ultra thin metal film, vaccum deposited on to a thin
dielectric or semiconductor pellicle. The pellicle is supported at
its edges by a suitable substrate material, on which are formed
metal bonding pads. Electrical connection is made between the
radiation sensitive detector element and the bonding pads by means
of thin metal contacts.
A hole, or alternatively a channel or slot pattern is drilled
through or etched into the substrate, and the pellicle is
freely-supported over the recess thus formed. A single detector
element in a multi-element detector array has the pellicle
supported over an etched channel, and a metal film interconnect
joins the detector contacts to the bonding pads.
The detector or detector array is mounted inside a microcircuit
package fitted with an infrared transmitting window. Depending on
the desired performance, the package is either evacuated or filled
to a predetermined pressure with a selected gas.
In order however to fully understand the nature of the invention,
embodiments thereof will now be described with reference to the
accompanying drawings to which however the invention is not
necessarily limited.
In the drawings:
FIG. 1 is a schematic plan of a single element metal film bolometer
detector according to the invention,
FIG. 2 is a side elevation of same,
FIG. 3 and FIG. 4 are corresponding views of metal film bolometer
detector array.
FIG. 5 gives details showing the form that the substrates can take,
showing at A holes, channels and slots formed by diamond drilling,
spark erosion, abrasive or laser milling, at B formed by isotropic
etch and laser drill venting, at C formed by an anisotropic etch,
and at D formed by an anisotropic back-etched substrate.
In the drawings the substrate is designated 5, the pellicle 6, the
detector element 7, the thin film contacts 8 and the bonding pads
or, as shown in the figures interconnects 9. The holes are
designated 10.
In FIGS. 3 and 4 of the drawings is shown how an array can be
constructed by linearly extending the substrate 5 and the hole 10
and placing the pellicle 6 over the slot (or slots) thus formed
along the length of the substrate.
The substrate can be prepared as follows: Depending on the chosen
application and facilities available the substrate material may be
an aluminium oxide ceramic, a glass, an insulated metal or a
silicon slice. Silicon slices (wafers) are either thermally
oxidised or coated with a thin film dielectric material, to provide
electrical insulation.
Aluminium oxide (alumina) ceramic substrates are prepared by
grinding to the desired shape, and holes for the pellicle are
formed with a high speed diamond drill.
Glass substrates are ground and polished and the holes, channels or
slots are defined by conventional photolithographic methods, using
hydrofluoric acid etchant. Both quartz and borosilicate type
glasses may be used.
Metal substrates are prepared by conventional machining techniques,
either singly or in large repeated patterns which are subsequently
separated. The metals preferred are brass and aluminium. The
surface of the metal is polished and coated as appropriate with an
insulating layer by means of anodisation, chemical vapour
deposition, thermal deposition or sputter deposition. The standard
surface coating is sputter deposited silicon dioxide. An insulating
polymer coating may be used in some designs.
Polycrystalline or single crystal silicon substrates are furnished
with holes, channels or slots by means of conventional
photolithography. Both thermal oxide and metal film masks are
employed, depending on the etchant. Large numbers of substrates are
prepared on a single silicon wafer, using step-and-repeat
photographic artwork.
The patterns are etched in silicon substrates with either isotropic
or anisotropic etchants, the latter applying to single crystal
material having a (1,0,0) or (1,1,0) surface orientation. Good
results have been achieved with conventional nitric/hydrofluoric
acid isotropic etchants, and a hydrazine anisotropic etchant.
Holes, channels or slots extend through the thickness of the
substrate, but where this requirement is not convenient or
practicable, laser-drilled venting holes are provided, assisted as
required by rear surface etching to reduce the drill depth as shown
in FIG. 5B.
It is to be noted that spark erosion, abrasive and laser milling
are alternative methods of generating hole or slot patterns in
substrate materials, and have been successfully employed for
alumina and silicon substrates.
Anisotropic etching of (1,0,0) surface orientation silicon is
illustrated in FIG. 5C. The use of an anisotropic rear surface etch
for venting purposes is optional to isotropic etching.
A one-step anisotropic rear etch process can be used, as shown in
FIG. 5D. It is to be noted that the optional use of (1,1,0) surface
orientation silicon results in channels which have vertical side
walls.
Which technique should be employed depends on the user's
requirements. Thus aluminium oxide, glass or metal substrates are
suitable for small batch preparations of single element detectors,
whilst silicon substrates are admirably suited to large scale
production. Furthermore in the latter case isotropic etchants are
adequate or indeed necessary for some applications, whereas
anisotropic etchants are preferred for high definition pattern
generation.
Bonding pads are applied either by vacuum deposition through
precision mechanical masks, or by vacuum deposition and/or
electroplating followed by pattern etching using photographic
masks. In both cases, large numbers of substrates are processed
during a single operation.
When metal film masks are used for hole and slot etching in silicon
substrates, the same film may be used for bonding pads, and the
substrate then cut to the desired size using a microcircuit dicing
saw.
The bonding pad material is either aluminium or gold, depending on
the application. Gold film deposition is preceded by deposition of
a thin metal bonding and diffusion barrier film (or films), such as
chromium or nichrome followed by nickel, or a single layer of
tantalum or molybdenem, or either tantalum or titanium followed by
platinum.
Three general methods have been devised for pellicle preparation,
and are employed optionally to meet the user's applications. In the
first of these makes use of a polymer film approximately 50 nm
thick, which is solvent cast on a glass surface, then separated on
water and collected on a metal or epoxy resin annulus. The polymer
membrane is then pressed on to the substrate surface and adhered by
directing a humid jet of air on to the membrane surface. This
technique is employed for simple, low cost detectors of moderate
performance where heat treatment cycles are not required.
The usual polymer material is cellulose nitrate, but numerous
materials (notably polyvinyl chloride and polyvinyl formal) have
been successfully employed.
Bolometer detectors comprised of a thin film of nickel, gold or
bismuth, vacuum deposited on to a cellulose nitrate pellicle have
previously been reported, (references 1 to 3); however, the
application of polymer pellicles, in conjunction with the above
substrate preparation techniques, has not been described.
In the second method, the pellicle is a thin inorganic dielectric
or semiconductor film prepared either by anodisation, or by vacuum
deposition on to a metal film or polymer membrane. The desired
pellicle shape is generated by mechanical or photographic masking
techniques and the metal or polymer backing is removed by
preferential etching, solvents or (in the case of polymers) either
oven stoving or plasma ashing. The dielectric or semiconductor film
is usually pressed on to the substrate prior to removal of the
backing, but this process is optional and will depend on the
individual detector design.
The pellicle is precisely located on the substrate by means of a
mechanical alignment jig.
Aluminium oxide, silicon dioxide, silicon monoxide, cryolite,
germanium and silicon pellicles have been successfully prepared by
this method. The thickness of the pellicles is less than 50 nm, and
typically 25 nm for most applications.
Bolometer detectors which employ anodised aluminium oxide pellicles
have been described (references 4 and 5); but not as described
herein. Nor have pellicle thicknesses less than 50 nm been
reported, regardless of the preparation method.
The third and most advanced technique for pellicle preparation is a
method by which the pellicle material is deposited by vacuum or
chemical vapour deposition techniques anodisation or a
plasma-assisted process, directly onto a silicon substrate. Holes,
channels or slots are then etched into the rear surface of the
silicon wafer, rendering the pellicle freely-supporting.
Silicon nitride, aluminium oxide and elemental silicon film
pellicles have been prepared by this method. Note that when the
surface of the silicon substrate is provided with an insulator
layer, e.g., thermal oxide, this layer must also be removed.
Electrical interconnections and detector contacts are prepared by
similar processes described above for bonding pads and in some
designs are simple extensions of the bonding pads.
The detector elements are ultra thin films of gold, nickel,
palladium, iridium or platinum. Selection depends on the desired
application. For the best performance, the high melting point noble
metals are preferred. Detailed processing steps also depend on the
selected metal. Three methods of preparation are identified. Each
method may be used for the manufacture of both single element
detectors and detector arrays.
The first method of detector preparation is employed for all high
melting point metal detector films, including nickel and palladium,
and with inorganic pellicles.
Processed substrates, complete with bonding pads, interconnections,
contacts and detector pellicle, are inserted in a deposition jig
fitted with a precision micromechanical evaporation mask. The
detector elements are deposited through the mask by means of high
vacuum thermal evaporation.
Prior to deposition the substrate is thermally outgassed in vacuo.
The substrate may also be heated during deposition, but this step
is optional and not necessary for some evaporants. The vacuum
coating unit should have excellent backstreaming protection and be
capable of an ultimate pressure of 1.times.10.sup.-4 Pa. During
deposition, the pressure and deposition rate should be such that
the ratio of arrival rate of metal atoms to the impingement rate of
residual gas species is large.
Deposition is terminated at a predetermined electrical resistance,
measured using a monitor substrate fitted with electrical probes.
The film thickness is also monitored during deposition. A brief
post deposition bake in vacuo is beneficial for some materials, but
is an optional processing step.
Following removal from the vacuum chamber, the detector is annealed
in a selected gas. The temperature depends on the particular
detector metal, but is typically 250.degree. C. Nickel films are
baked in hydrogen and the noble metal films are annealed in air or
an inert gas. The annealing process encourages recrystallization,
reduces structural defects, and removes impurities, thereby
stabilising the electrical resistance and enhancing the temperature
coefficient of resistance.
The selected resistance and thickness of the deposited films are
characteristics of the individual metal, and the final detector
resistance is influenced by the processing parameters. Preparation
parameters are chosen to obtain a final sheet resistance in the
range 150 to 250 ohm per square, corresponding to the region of
maximum infrared absorption, which for metal films is 50%. The
thickness is less than 10 nm, and for higher melting point metals
such as platinum and iridium the typical thickness is 5 nm.
This method provides for the preparation of bolometer detectors
which have optimum electrical, optical and thermophysical
properties. Since both the metal film and pellicle are extremely
thin, the thermal resistance of the detector is exceptionally high,
whilst the thermal capacitance is small. The method thereby
produces detectors of high sensitivity, fast speed of response, and
flat frequency response characteristic up to the frequency
cutoff.
A second optional method for low temperature preparations is mainly
employed for lower melting point metals such as gold, and for
polymer pellicles. The same equipment described above is used;
however the substrate is not heated during processing. The method
has also been employed with inorganic dielectric pellicles.
An electrical `forming` current is applied to the detector element
during deposition, and is subsequently maintained until the
electrical resistance is stable. Although the resistance is
continuously monitored, control of thickness and deposition rate is
the major consideration. The desired thickness is a characteristic
of the specific metal, and for gold is in the range 15 to 20
nm.
Gold film bolometers have been extensively reported, but the method
of attaining optimum detective properties in films less than 20 nm
thick, by means of a precise control of thickness and deposition
rate in conjunction with a forming current, form part of this
invention which relates to improved technology and attainment of
optimum material parameters.
In the third method of detector preparation evaporation masks are
not used; however preparation parameters, including annealing
cycles, are similar to those described above for high melting point
metals. The detector element may be deposited by high vacuum
thermal evaporation or by sputtering.
The detector metal is deposited onto the surface of a silicon wafer
precoated with the desired pellicle material. Detector elements are
defined by conventional microcircuit photolithography. Contacts,
interconnects and bonding pads are also formed by pattern etching
using photographic masks.
By any of the above three methods, detector elements of size
ranging from several millimetres to less than 0.05 mm can be
fabricated.
Following detector processing, the substrate is rear-etched by the
method described earlier herein.
A number of detector or detector arrays are prepared on a single
silicon wafer and these are separated using a microcircuit dicing
saw, or by rear surface etching.
Processed detectors are mounted in microcircuit packages fitted
with vacuum-tight infrared transmitting windows. Electrical
connection is made to detectors by conventional microcircuit wire
bonding methods.
The final package seal is carried out in vacuo or at a
predetermined pressure in a selected gas. Optionally, the package
is fitted with an evacuation tube and, after lid sealing, this tube
is used to evacuate the package and back-fill with the selected
gas. The tube is then pinched off.
A dedicated package sealing system has been constructed which
facilitates assembly and sealing of the package components entirely
within a vacuum-tight enclosure. This equipment is fitted with
outgassing and sealing apparatus, and provision is made for precise
control of both temperature and absolute gas pressure. Detector
responsivity and speed of response can be monitored during the
sealing process.
The range of gases used include nitrogen, argon, Freon 12, Freon 22
and xenon. The control of both pressure and gas type allows the
user to trade between detector sensitivity and speed of response,
and is an integral component of preparation technique.
Encapsulation of bolometer detectors in vacuo or an inert gas is an
established technique. However, the deliberate adjustment of
pressure, in conjunction with a selected gas of known
thermophysical properties, in order to attain a wide range of
detective performance suitable to the user's needs, is a novel
development.
* * * * *